Digital cameras comprise imaging optics which are adapted to focus the image of a target onto an image sensor. The image sensor comprises an array of light sensitive pixels. The plurality of signals provided by the individual pixels constitute the digital image of the target. To the first approximation, a signal provided by each individual pixel is proportional to a time period between resetting or activation of said pixel and shifting of said signal (e.g. charge) to a shift register or to a memory.
A typical low-cost image sensor does not comprise charge shift registers and it cannot hold and store the signals. Thus, when the reading of all signals can not take place simultaneously, the signals obtained from the individual pixels correspond to different time intervals. This may lead to a distortion of the recorded image, image artifacts, when the digital camera or the target is moving.
The problem may be remedied by controlling the optical exposure of all pixels substantially simultaneously by using a so-called global shutter. The signal provided by an individual pixel remains substantially constant after closing the global shutter, and the obtained image corresponds to simultaneous exposure even though the electrical reading of the pixels would not take place simultaneously.
Traditional mechanical global shutters exhibit several drawbacks, e.g. complex mechanical structure, large size and relatively high power consumption. Therefore, mechanically implemented global shutters are not optimal for imaging systems integrated in small portable devices.
U.S. Pat. No. 4,701,021 discloses a light beam intensity modulator for controlled attenuation of a light beam. Said modulator is based on a plurality of successive cells. Each of said cells comprises a capillary space between two transparent plates, and an electric field producing means for applying an electric field between said transparent plates to control the displacement of a partially transparent fluid between said capillary space and a reservoir. Light beam intensity is modulated by moving said partially transparent fluid into the line of the light beam in one or more of the cells. According to the teachings of said US publication, the attenuation provided by a single cell is in the order of 10-90%. Further, according to the teachings of said US publication, the operating time of said cells is in the order of 100 ms.
Hence, such a light beam intensity modulator is not applicable as an effective optical shutter in a camera because of insufficient speed and light attenuation capabilities.
Clearly, there is still room for further development in the field of global optical shutters to create a solution that does not suffer from those disadvantages of the solutions.